Treatment of insulin resistance

ABSTRACT

Disclosed are a method of for treating insulin resistance and a method identifying a compound for treating insulin resistance.

BACKGROUND

Insulin resistance or glucose intolerance is a condition characterized by the body's inability to properly use insulin or blood sugar. In this condition, normal amounts of insulin are inadequate to produce a normal insulin response from fat, muscle, or liver cells. Insulin resistance in fat cells results in hydrolysis of stored triglycerides, which elevates free fatty acids in the blood plasma. Insulin resistance in muscle reduces glucose uptake whereas insulin resistance in liver reduces glucose storage, with both effects serving to elevate blood glucose. High plasma levels of insulin and glucose due to insulin resistance often lead to the metabolic syndrome, such as abdominal obesity, atherogenic dyslipidemia, elevated blood pressure, prothrombotic state (e.g., high fibrinogen or plasminogen activator inhibitor-1 in the blood), and proinflammatory state (e.g., elevated C-reactive protein in the blood). People with the metabolic syndrome are at increased risk of cardiovascular diseases and type 2 diabetes. The metabolic syndrome has become increasingly common in the United States. There is a need for effective treatment of insulin resistance.

SUMMARY

This invention relates to treating insulin resistance and identifying a compound for treating insulin resistance.

In one aspect, the invention features a method of identifying a compound for treating insulin resistance. The method includes contacting a first cell expressing an acid-sensing ion channel 3 (ASIC3) with a test compound, and determining an expression or activity level of the acid-sensing ion channel 3 in the cell. The test compound is determined to be a candidate compound for treating insulin resistance if the expression or activity level is lower than that determined in the same manner from a second cell except that the second cell is not contacted with the compound. The activity of the acid-sensing ion channel 3 can be determined by electrophysical analysis. The ion channel activity of the acid-sensing ion channel 3 can be determined by whole-cell patch recording, a voltage-sensitive dye, an ion-sensitive dye, or a cytotoxicity assay. The method can further include administering the candidate compound to a non-human animal to confirm an efficacy thereof to treat insulin-resistance. The first cell or second cell can also be a non-neuronal and AdiproR2⁺ cell. Examples of the cells include NIH3T3 cells, NIH3T3 L1 cells, differentiated NIH3T3 L1 cells, adipose cells, and fibroblast cells. The first cell or second cell can be positive for one or more of FAS, FGF10, aP2, ETO, PPARγ, and AdipoR1. In one example, the first or second cell is an adipose tissue cell, such as a white adipose tissue cell.

In another aspect, the invention features a method of treating insulin resistance, The method includes identifying a subject suffering from or being at risk for developing insulin resistance; inhibiting acid-sensing ion channel 3 in the subject; determining an expression or activity level of the acid-sensing ion channel 3 in a sample obtained from the subject before or after the inhibiting step; and comparing the level with a control level to confirm inhibition. The inhibiting step can be conducted by administering to the subject an effective amount of a polypeptide that binds to the acid-sensing ion channel 3 or a nucleic acid that decreases the expression level of the acid-sensing ion channel 3. The control level can be obtained from a normal subject.

The invention further features a method of treating insulin resistance. The method includes identifying a subject suffering from or being at risk for developing insulin resistance, and administering to the subject an effective amount of an inhibitor of an acid-sensing ion channel 3. The inhibitor can be a polypeptide that binds to the acid-sensing ion channel 3 or a nucleic acid that decreases the expression level of the acid-sensing ion channel 3. For example, the inhibitor is an antibody or an antisense nucleic acid or an RNAi agent The control level is obtained from a normal subject.

The invention also features a method for acute treatment of insulin resistance. The method includes identifying a subject suffering from or being at risk for developing insulin resistance, and administering to the subject, within an hour of food ingestion, an effective amount of an inhibitor of an acid-sensing ion channel 3.

The inhibitor can be a polypeptide that binds to the acid-sensing ion channel 3 or a nucleic acid that decreases the expression level of the acid-sensing ion channel 3. For example, the inhibitor is an antibody or an antisense nucleic acid or an RNAi agent. The control level is obtained from a normal subject.

The details of one or more embodiments of the invention are set forth in the accompanying description below. Other features, objects, and advantages of the invention will be apparent from the detailed description, and from the claims.

DETAILED DESCRIPTION

This invention is based, at least in part, on the unexpected discoveries that ASIC3−/− mice were protected against age-dependent glucose intolerance with enhanced insulin sensitivity and that age-dependent glucose intolerance was associated with the up-regulation of ASIC3 expressed in white adipose tissue (WAT) but not sensory neurons.

Acid-sensing ion channels (ASICs) mediate inward currents and depolarize cells when the extracellular pH drops (Krishtal, 2003, Trends Neurosci. 26, s477-483). They belong to the epithelial sodium channel/degenerin superfamily, which are characterized by two membrane-spanning domains with intracellular N- and C-termini and a large extracellular loop (Waldmann and Laadunski, 1998, Curr. Opin. Neurobiol. 8, 418-424 and Kellenberger and Schild, 2002, Physiol. Rev. 82, 735-767). There are seven ASIC subunits, including ASIC1a, ASIC1b, ASIC2a, ASIC2b, ASIC3, ASIC4, and ASIC5, which are encoded by five genes. Multiple subunits assemble to form a functional ion channel. Homomeric or heteromeric ASICs has various sensitivity to extracellular pH, ranging from near physiological pH (˜pH 7.0 for ASIC3) to very acidic status (˜pH 5.0 for ASIC2a). ASICs express throughout neurons of mammalian central and peripheral nervous systems (Krishtal, 2003, Trends Neurosci. 26, 477-483). Accordingly, the role of ASICs has been implicated in detecting tissue acidosis, ischemia, and modulating synaptic activity (Molliver et al., 2005, Molecular Pain 1, 35, and Xiong et al, 2004, Cell 118, 687-698).

ASIC3 is the most sensitive acid-sensing ion channel (pH_(0.5) activation ˜6.7) and predominantly expressed in primary sensory neurons, especially in metaboreceptive sensory neurons or ischemia-sensing neurons (Waldmann and Lasdunski, 1998, Curr. Opin. Neurobiol. 8, 418-424). It responds better to lactic acid than to other acids to depolarize ischemia-sensing neurons. Therefore, ASIC3-expressing neurons might function as metaboreceptors to sense the anaerobic metabolism of tissues and trigger acid-linked pain sensation in muscle and heart (Molliver et al., 2005, Molecular Pain 1, 35).

The invention features a method for identifying an inhibitor of ASIC3 for treating insulin resistance. An ASIC3 inhibitor, which reduces ASIC3's expression level or channel activity in a statistically significant manner, can be identified according to the methods described below.

Candidate compounds to be screened (e.g., proteins, peptides, peptidomimetics, peptoids, antibodies, small molecules, or other drugs) can be obtained using any of the numerous approaches in combinatorial library methods known in the art. Such libraries include: peptide libraries, peptoid libraries (libraries of molecules having the functionalities of peptides, but with a novel, non-peptide backbone that is resistant to enzymatic degradation); spatially addressable parallel solid phase or solution phase libraries; synthetic libraries obtained by deconvolution or affinity chromatography selection; and the “one-bead one-compound” libraries. See, e.g., Zuckermann et al. 1994, J. Med. Chem. 37:2678-2685; and Lam, 1997, Anticancer Drug Pes. 12:145. Examples of methods for the synthesis of molecular libraries can be found in, e.g., DeWit et al., 1993, PNAS USA 90:6909; Erb et al., 1994, FNAS USA 91:11422; Zuckermann et al., 1994, J. Med. Chem. 37:2678; Cho et al., 1993, Science 261:1303; Carrell et al., 1994, Angew. Chem. Int. Ed. Engl. 33:2059; Carrell et al., 1994, Angew. Chem. Int. Ed. Engl. 33:2061; and Gallop et al., 1994 J. Med. Chem. 37:1233. Libraries of compounds may be presented in solution (e.g., Houghten, 1992, Biotechniques 13:412-421), or on beads (Lam, 1991, Nature 354:82-84), chips (Fodor, 1993, Nature 364:555-556), bacteria (U.S. Pat. No. 5,223,409), spores (U.S. Pat. No. 5,223,409), plasmids (Cull et al., 1992, PNAS USA 89:1865-1869), or phages (Scott and Smith 1990, Science 249:386-390; Devlin, 1990, Science 249:404-406; Cwirla et al., 1990, PNAS USA 87:6378-6382; Felici 1991, J. Mol. Biol. 222:301-310; and U.S. Pat. No. 5,223,409).

To identify an ASIC3 inhibitor, one can contact a candidate compound with a system containing an ASIC3 gene or polypeptide. The system can be a cell-free system or a cell-containing system, e.g., an in vitro cell line model or an in vivo animal model. In a cell-containing system, cells can naturally express the ASIC3 gene, or can be modified to express a recombinant nucleic acid. The recombinant nucleic acid can contain the ASIC3 gene coding region fused to a heterologous promoter or an ASIC3 gene promoter sequence fused to a reporter gene. One then measures the expression level of the channel activity of ASIC3 polypeptide-containing channels.

An ASIC3 polypeptide described above refers to a full-length ASIC3 polypeptide or its functional equivalent. A functional equivalent refers to a polypeptide derived from the ASIC3 polypeptide, e.g., a fusion polypeptide or a polypeptide having one or more point mutations, insertions, deletions, truncations, or combination thereof. This polypeptide retains substantially activity of the ASIC3 polypeptide to formal functional channel, i.e., the ability to mediate inward currents and depolarize cells when the extracellular pH drops, as described above. Shown below are the amino acid and nucleotide sequences of various transcription variants and single nucleotide polymorphism (SNP) variants of ASIC3.

Transcript variant 1 (isoform a): 531 a.a. ACCESSION NP_004760 (SEQ ID NO: 1) 1 MKPTSGPEEA RRPASDIRVF ASNCSMHGLG HVFGPGSLSL RRGMWAAAVV LSVATFLYQV 61 AERVRYYREF HHQTALDERE SHRLIFPAVT LCNINPLRRS RLTPNDLHWA GSALLGLDPA 121 EHAAFLRALG RPPAPPGFMP SPTFDMAQLY ARAGHSLDDM LLDCRFRGQP CGPENFTTIF 181 TRMGKCYTFN SGADGAELLT TTRGGMGNGL DIMLDVQQEE YLPVWRDNEE TPFEVGIRVQ 241 IHSQEEPPII DQLGLGVSPG YQTFVSCQQQ QLSPLPPPWG DCSSASLNPN YEPEPSDPLG 301 SPSPSPSPPY TLMGCRLACE TRYVARKCGC RMVYMPGDVP VCSPQQYKNC AHPAIDAMLR 361 KDSCACPNPC ASTRYAKELS MVRIPSRAAA RFLARKLNRS EAYIAENVLA LDIFFEALNY 421 ETVEQKKAYE MSELLDGIGG QMGLFIGASL LTILEILDYL CEVFRDKVLG YFWNRQHSQR 481 HSSTNLLQEG LGSHRTQVPH LSLGPRPPTP PCAVTKTLSA SHRTCYLVTQ L Transcript variant 1 (isoform a): 1746 bp ACCESSION NM_004769 (SEQ ID NO: 2) 1 agaattcggc acgacggggt tctggccatg aagcccacct caggcccaga ggaggcccgg 61 cggccagcct cggacatccg cgtgttcgcc agcaactgct cgatgcacgg gctgggccac 121 gtcttcgggc caggcagcct gagcctgcgc cgggggatgt gggcagcggc cgtggtcctg 181 tcagtggcca ccttcctcta ccaggtggct gagagggtgc gctactacag ggagttccac 241 caccagactg ccctggatga gcgagaaagc caccggctca tcttcccggc tgtcaccctg 301 tgcaacatca acccactgcg ccgctcgcgc ctaacgccca acgacctgca ctgggctggg 361 tctgcgctgc tgggcctgga tcccgcagag cacgccgcct tcctgcgcgc cctgggccgg 421 ccccctgcac cgcccggctt catgcccagt cccacctttg acatggcgca actctatgcc 481 cgtgctgggc actccctgga tgacatgctg ctggactgtc gcttccgtgg ccaaccttgt 541 gggcctgaga acttcaccac gatcttcacc cggatgggaa agtgctacac atttaactct 601 ggcgctgatg gggcagagct gctcaccact actaggggtg gcatgggcaa tgggctggac 661 atcatgctgg acgtgcagca ggaggaatat ctacctgtgt ggagggacaa tgaggagacc 721 ccgtttgagg tggggatccg agtgcagatc cacagccagg aggagccgcc catcatcgat 781 cagctgggct tgggggtgtc cccgggctac cagacctttg tttcttgcca gcagcagcag 841 ctgagcttcc tgccaccgcc ctggggcgat tgcagttcag catctctgaa ccccaactat 901 gagccagagc cctctgatcc cctaggctcc cccagcccca gccccagccc tccctatacc 961 cttatggggt gtcgcctggc ctgcgaaacc cgctacgtgg ctgccaagtg cggctgccga 1021 atggtgtaca tgccaggcga cgtgccagtg tgcagccccc agcagtacaa gaactgtgcc 1081 cacccggcca tagatgccat gcttcgcaag gactcgtgcg cctgccccaa cccgtgcgcc 1141 agcacgcgct acgccaagga gctctccatg gtgcggatcc cgagccgcgc cgccgcgcgc 1201 ttcctggccc ggaagctcaa ccgcagcgag gcctacatcg cggagaacgt gctggccctg 1261 gacatcttct ttgaggccct caactatgag accgtggagc agaagaaggc ctatgagatg 1321 tcagagctgc ttggtgacat tgggggccag atggggctgt tcatcggggc cagcctgctc 1381 accatcctcg agatcctaga ctacctctgt gaggtgttcc gagacaaggt cctgggatat 1441 ttctggaacc gacagcactc ccaaaggcac tccagcacca atctgcttca ggaagggctg 1501 ggcagccatc gaacccaagt tccccacctc agcctgggcc ccagacctcc cacccctccc 1561 tgtgccgtca ccaagactct ctccgcctcc caccgcacct gctaccttgt cacacagctc 1621 tagacctgct gtctgtgtcc tcggagcccc gccctgacat cctggacatg cctagcctgc 1681 acgtagcttt tccgtcttca ccccaaataa agtcctaatg catcaaaaaa aaaaaaaaaa 1741 aaaaaa Transcript variant 2 (isoform b): 549 a.a. ACCESSION NP_064717 (SEQ ID NO: 3) 1 MKPTSGPEEA RRPASDIRVF ASNCSMHGLG HVFGPGSLSL RRGMWAAAVV LSVATFLYQV 61 AERVRYYREF HHQTALDERE SHRLIFPAVT LCNINPLRRS RLTPNDLHWA GSALLGLDPA 121 EHAAFLRALG RPPAPPGFMP SPTFDMAQLY ARAGHSLDDM LLDCRFRGQP CGPENFTTIF 181 TPMGKCYTFN SGADGAELLT TTRGGMGNGL DIMLDVQQEE YLPVWRDNEE TPPEVGIEVQ 241 IRSQEEPPII DQLGLGVSPG YQTFVSCQQQ QLSFLPPPWG DCSSASLNPN YEPEPSDPLG 301 SPSPSPSPPY TLMGCRLACE TRYVARKCGC RMVYMPGDVP VCSPQQYKNC AHPAIDAMLR 361 KDSCACPNPC ASTRYAKELS MVRIPSRAAA RFLARKLNRS EAYIAENVLA LDIFFEALNY 421 ETVEQKKAYE MSELLGDIGG QMGLFIGASL LTILEILDYL CEVFRDKVLG YFWNRQHSQR 481 HSSTNLLQEG LGSHRTQVPH LSLGPSTLLC SEDLPPLPVP SPRLSPPPTA PATLSHSSRP 541 AVCVLGAPP Transcript variant 2 (isoform b): 1766 bp ACCESSION NM_020321 (SEQ ID NO: 4) 1 agaattcggc acgacggggt tctggccatg aagcccacct caggcccaga ggaggcccgg 61 cggccagcct cggacatccg cgtgttcgcc agcaactgct cgatgcacgg gctgggccac 121 gtcttcgggc caggcagcct gagcctgcgc cgggggatgt gggcagcggc cgtggtcctg 181 tcagtggcca ccttcctcta ccaggtggct gagagggtgc gctactacag ggagttccac 241 caccagactg ccctggatga gcgagaaagc caccggctca tcttcccggc tgtcaccctg 301 tgcaacatca acccactgcg ccgctcgcgc ctaacgccca acgacctgca ctgggctggg 361 tctgcgctgc tgggcctgga tcccgcagag cacgccgcct tcctgcgcgc cctgggccgg 421 ccccctgcac cgcccggctt catgcccagt cccacctttg acatggcgca actctatgcc 481 cgtgctgggc actccctgga tgacatgctg ctggactgtc gcttccgtgg ccaaccttgt 541 gggcctgaga acttcaccac gatcttcacc cggatgggaa agtgctacac atttaactct 601 ggcgctgatg gggcagagct gctcaccact actaggggtg gcatgggcaa tgggctggac 661 atcatgctgg acgtgcagca ggaggaatat ctacctgtgt ggagggacaa tgaggagacc 721 ccgtttgagg tggggatccg agtgcagatc cacagccagg aggagccgcc catcatcgat 781 cagctgggct tgggggtgtc cccgggctac cagacctttg tttcttgcca gcagcagcag 841 ctgagcttcc tgccaccgcc ctggggcgat tgcagttcag catctctgaa ccccaactat 901 gagccagagc cctctgatcc cctaggctcc cccagcccca gccccagccc tccctatacc 961 cttatggggt gtcgcctggc ctgcgaaacc cgctacgtgg ctcggaagtg cggctgccga 1021 atggtgtaca tgccaggcga cgtgccagtg tgcagccccc agcagtacaa gaactgtgcc 1081 cacccggcca tagatgccat gcttcgcaag gactcgtgcg cctgccccaa cccgtgcgcc 1141 agcacgcgct acgccaagga gctctccatg gtgcggatcc cgagccgcgc cgccgcgcgc 1201 ttcctggccc ggaagctcaa ccgcagcgag gcctacatcg cggagaacgt gctggccctg 1261 gacatcttct ttgaggccct caactatgag accgtggagc agaagaaggc ctatgagatg 1321 tcagagctgc ttggtgacat tgggggccag atggggctgt tcatcggggc cagcctgctc 1381 accatcctcg agatcctaga ctacctctgt gaggtgttcc gagacaaggt cctgggatat 1441 ttctggaacc gacagcactc ccaaaggcac tccagcacca atctgcttca ggaagggctg 1501 ggcagccatc gaacccaagt tccccacctc agcctgggcc ccagcactct gctctgttcc 1561 gaagacctcc cacccctccc tgtgccgtca ccaagactct ctccgcctcc caccgcacct 1621 gctaccttgt cacacagctc tagacctgct gtctgtgtcc tcggagcccc gccctgacat 1681 cctggacatg cctagcctgc acgtagcttt tccgtcttca ccccaaataa agtcctaatg 1741 catcaaaaaa aaaaaaaaaa aaaaaa Transcript variant 3 (isoform c): 543 a.a. ACCESSION NP_064718 (SEQ ID NO: 5) 1 MKPTSGPEEA RRPASDIRVF ASNCSMHGLG HVFGPGSLSL RRGMWAAAVV LSVATFLYQV 61 AERVRYYREF HHQTALDERE SHRLIFPAVT LCNINPLRRS RLTPNDLHWA GSALLGLDPA 121 EHAAFLRALG RPPAPPGFMP SPTFDMAQLY ARAGHSLDDM LLDCRFRGQP CGPENFTTIF 181 TRMGKCYTFN SGADGAELLT TTRGGMGNGL DIMLDVQQEE YLPVWRDNEE TPFEVGIRVQ 241 IHSQEEPPII DQLGLGVSPG YQTFVSCQQQ QLSFLPPPWG DCSSASLNPN YEPEPSDPLG 301 SPSPSPSPPY TLMGCRLACE TRYVARKCGC RMVYMPGDVP VCSPQQYKNC AHPAIDAMLR 361 KDSCACPNPC ASTRYAKELS MVRIPSRAAA RFLARKLNRS EAYIAENVLA LDIFFEALNY 421 ETVEQKKAYE MSELLGDIGG QMGLFIGASL LTILEILDYL CEVFRDKVLG YFWNRQHSQR 481 HSSTNLTSHP SLCRHQDSLR LPPHLLPCHT ALDLLSVSSE PRPDILDMPS LHVAFPSSPQ 541 IKS Transcript variant 3 (isoform c): 1671 bp ACCESSION NM_020322 (SEQ ID NO: 6) 1 agaattcggc acgacggggt tctggccatg aagcccacct caggcccaga ggaggcccgg 61 cggccagcct cggacatccg cgtgttcgcc agcaactgct cgatgcacgg gctgggccac 121 gtcttcgggc caggcagcct gagcctgcgc cgggggatgt gggcagcggc cgtggtcctg 181 tcagtggcca ccttcctcta ccaggtggct gagagggtgc gctactacag ggagttccac 241 caccagactg ccctggatga gcgagaaagc caccggctca tcttcccggc tgtcaccctg 301 tgcaacatca acccactgcg ccgctcgcgc ctaacgccca acgacctgca ctgggctggg 361 tctgcgctgc tgggcctgga tcccgcagag cacgccgcct tcctgcgcgc cctgggccgg 421 ccccctgcac cgcccggctt catgcccagt cccacctttg acatggcgca actctatgcc 481 cgtgctgggc actccctgga tgacatgctg ctggactgtc gcttccgtgg ccaaccttgt 541 gggcctgaga acttcaccac gatcttcacc cggatgggaa agtgctacac atttaactct 601 ggcgctgatg gggcagagct gctcaccact actaggggtg gcatgggcaa tgggctggac 661 atcatgctgg acgtgcagca ggaggaatat ctacctgtgt ggagggacaa tgaggagacc 721 ccgtttgagg tggggatccg agtgcagatc cacagccagg aggagccgcc catcatcgat 781 cagctgggct tgggggtgtc cccgggctac cagacctttg tttcttgcca gcagcagcag 841 ctgagcttcc tgccaccgcc ctggggcgat tgcagttcag catctctgaa ccccaactat 901 gagccagagc cctctgatcc cctaggctcc cccagcccca gccccagccc tccctatacc 961 cttatggggt gtcgcctggc ctgcgaaacc cgctacgtgg ctcggaagtg cggctgccga 1021 atggtgtaca tgccaggcga cgtgccagtg tgcagccccc agcagtacaa gaactgtgcc 1081 cacccggcca tagatgccat gcttcgcaag gactcgtgcg cctgccccaa cccgtgcgcc 1141 agcacgcgct acgccaagga gctctccatg gtgcggatcc cgagccgcgc cgccgcgcgc 1201 ttcctggccc ggaagctcaa ccgcagcgag gcctacatcg cggagaacgt gctggccctg 1261 gacatcttct ttgaggccct caactatgag accgtggagc agaagaaggc ctatgagatg 1321 tcagagctgc ttggtgacat tgggggccag atggggctgt tcatcggggc cagcctgctc 1381 accatcctcg agatcctaga ctacctctgt gaggtgttcc gagacaaggt cctgggatat 1441 ttctggaacc gacagcactc ccaaaggcac tccagcacca atctgacctc ccacccctcc 1501 ctgtgccgtc accaagactc tctccgcctc ccaccgcacc tgctaccttg tcacacagct 1561 ctagacctgc tgtctgtgtc ctcggagccc cgccctgaca tcctggacat gcctagcctg 1621 cacgtagctt ttccgtcttc accccaaata aagtcctaat gcatcaaaaa a SNP Variants of ASIC3 Cluster Report: rs3192795

(SEQ ID NO:7) CTCCCTGTGCCGTCACCAAGACTCT [C/T] TCCGCCTCCCACCGCACC TGCTACC

Amino mRNA dbSNP Protein Codon acid Contig-->mRNA-->Protein orientation position Function allele residue pos pos NT_007914->NM_020321->NP_064717 forward 1595 nonsynonymous T Phe [F] 2 525 contig reference C Ser [S] 2 525 NT_007914->NM_020322->NP_064718 forward 1516 nonsynonymous T Phe [F] 1 499 contig reference C Leu [L] 1 499 Cluster Report: rs1864545

(SEQ ID NO:8) GAATATCTACCTGTGTGGAGGGACA [A/G] TGGTAGGGAGCACACAAA TGAGGCT

Amino mRNA dbSNP Protein Codon acid Contig-->mRNA-->Protein orientation position Function allele residue pos pos NT_007914->NM_004769->NP_004760 forward 704 nonsynonymous G Ser [S] 2 228 contig reference A Asn [N] 2 228 NT_007914->NM_020321->NP_064717 forward 704 nonsynonymous G Ser [S] 2 228 contig reference A Asn [N] 2 228 NT_007914->NM_020322->NP_064718 forward 704 nonsynonymous G Ser [S] 2 228 contig reference A Asn [N] 2 228

The expression level can be determined at either the mRNA level oral the protein level. Methods of measuring mRNA levels in a cell, a tissue sample, or a body fluid are well known in the art. To measure mRNA levels, cells can be lysed and the levels of mRNA in the lysates or in RNA purified or semi-purified from the lysates can be determined by, e.g., hybridization assays (using detectably labeled gene-specific DNA or RNA probes) and quantitative or semi-quantitative RT-PCR (using appropriate gene-specific primers). Alternatively, quantitative or semi-quantitative in situ hybridization assays can be carried out using tissue sections or unlysed cell suspensions, and detectably (e.g., fluorescent or enzyme) labeled DNA or RNA probes. Additional mRNA-quantifying methods include RNA protection assay (RPA) and SAGE.

Methods of measuring protein levels in a cell, a tissue sample, or a body fluid are also known in the art. Many such methods employ antibodies (e.g., monoclonal or polyclonal antibodies) that bind specifically to a target protein. In such assays, the antibody itself or a secondary antibody that binds to if can be detectably labeled. Alternatively, the antibody can be conjugated with biotin, and detectably labeled avidin (a polypeptide that binds to biotin) can be used to detect the presence of the biotinylated antibody. Combinations of these approaches (including “multi-layer sandwich” assays) can be used to enhance the sensitivity of the methodologies. Some of these protein-measuring assays (e.g., ELISA or Western blot) can be applied to body fluids or to lysates of cells, and others (e.g., immunohistological methods or fluorescence flow cytometry) applied to histological sections or unlysed cell suspensions. Methods of measuring the amount of label depend on the nature of the label and are well known in the art. Appropriate labels include radionuclides (e.g., ¹²⁵I, ¹³¹I, ³⁵S, ³H, or ³²P), enzymes (e.g., alkaline phosphatase, horseradish peroxidase, luciferase, or β-glactosidase), fluorescent moieties or proteins (e.g., fluorescein, rhodamine, phycoerythrin, GFP, or BFP), or luminescent moieties (e.g., Qdot™ nanoparticles supplied by the Quantum Dot Corporation, Palo Alto, Calif.). Other applicable assays include quantitative immunoprecipitation or complement fixation assays.

Methods of measuring ASIC3's ion channel activity are also known in the art. Examples of the methods include using whole-cell patch recording (Waldmann et al., J Biol Chem 272, 20975-20978 (1997); Voilley et al., J Neurosci 21, 8026-8033 (2001); Molliver et at. Molecular Pain 1:35 (2005); using voltage-sensitive dye (Felix et al., Assay Drug Dev Technol. 2, 260-268 (2004); Sharma et al., Biophys J 88, 3038-3049 (2005)), using ion-sensitive dyes and cytotoxicity assay (Weiser, J Neurosci Methods 137, 79-85 (2004).

To determine the ability of a candidate compound to inhibit ASIC3, one compares the level or activity obtained in the manner described above with a control level or activity obtained in the absence of the candidate compound. If the level or activity is lower than the control, the compound is identified as being effective for treating insulin resistance. One can further verify the efficacy of a compound thus-identified using an animal model. One can administer the compound to animal models and exam them according to the method describe below in the Example section or other standard techniques. Any statistically significant improvement of insulin sensitivity indicates the compound is a candidate for treating insulin resistance.

The invention also features methods for treating insulin resistance. A subject to be treated can be identified by standard diagnosing techniques for insulin resistance, such as diabetes. Optionally, the subject can then be examined for the gene expression or activity level of the ASIC3 polypeptide in adipose tissue cells (e.g., white adipose tissue cells) by methods described above. If the gene expression or channel activity level is higher in a sample from the subject than that in a sample from a normal person, the subject is a candidate for treatment with an effective amount of an ASIC3 inhibitor.

“Treating” refers to administration of a compound to a subject, who has insulin resistance, with the purpose to cure, alleviate, relieve, remedy, prevent, or ameliorate the disorder, the symptom of the disorder, the disease state secondary to the disorder, or the predisposition toward the disorder. An “effective amount” refers to an amount of the compound that is capable of producing a medically desirable result, e.g., as described above, in a treated subject. The treatment method can be performed in viva or ex vivo, alone or in conjunction with other drugs or therapy.

In an in viva approach, an ASIC3 inhibitor is administered to a subject. Generally, the compound is suspended in a pharmaceutically-acceptable carrier (e.g., physiological saline) and administered orally or by intravenous infusion, or injected or implanted subcutaneously, intramuscularly, intrathecally, intraperitoneally, intrarectally, intravaginally, intranasally, intragastrically, intratracheally, or intrapulmonarily. For treatment of insulin resistance, the compound can be delivered directly to white adipose tissues or surrounding tissues.

The dosage required depends on the choice of the route of administration; the nature of the formulation; the nature of the patient's illness; the subject's size, weight, surface area, age, and sex; other drugs being administered; and the judgment of the attending physician. Suitable dosages are in the range of 0.01-100 mg/kg. Variations in the needed dosage are to be expected in view of the variety of compounds available and the different efficiencies of various routes of administration. For example, oral administration would be expected to require higher dosages than administration by i.v. injection. Variations in these dosage levels can be adjusted using standard empirical routines for optimization as is well understood in the art. Encapsulation of the compound in a suitable delivery vehicle (e.g., polymeric microparticles or implantable devices) may increase the efficiency of delivery, particularly for oral delivery.

Examples of compounds that can be used to treat insulin resistance include polypeptides such as APETx2 (GTACSCGNSKGIYWFYRPSCPTDRGYTGSCRY FLGTCCTFAD) (SEQ ID NO:9) and its functional equivalents. A functional equivalent of APETx2 refers to a polypeptide derived from the APETx2 polypeptide, e.g., a fusion polypeptide or a polypeptide having one or more point mutations, insertions, deletions, truncations, or combination thereof. This polypeptide retains substantially activity of the APETx2 polypeptide, i.e., the ability to inhibit ASIC3 homomeric channels and ASIC3-containing heteromeric channels, as described in Diochot et al. EMBO J. 2004, 23(7): 1516-25.

The polypeptides can be synthesized using methods known in the art or be prepared using recombinant technology. For example, one can clone a nucleic acid encoding the polypeptide in an expression vector, in which the nucleic acid is operably linked to a regulatory sequence suitable for expressing the polypeptide in a host cell. One can then introduce the vector into a suitable host cell to express the polypeptide. The expressed recombinant polypeptide can be purified from the host cell by methods such as ammonium sulfate precipitation and fractionation column chromatography. See Goeddel, (1990) Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, Calif. A polypeptide thus prepared can be tested for its activity against ASIC3 homomeric channels and ASIC3-containing heteromeric channels according to the method described in the Example below or in Diochot et al. EMBO J. 2004, 23(7):1516-25.

Examples of compounds that can be used to treat insulin resistance also include antibodies that bind to polypeptides ASIC3 polypeptide. An “antibody” includes intact molecules as well as fragments thereof, such as Fab, F(ab′)2, Fv, scFv (single chain antibody), and dAb (domain antibody; Ward, et al. (1989) Nature, 341, 544). A derivative of an antibody refers to a protein or a protein complex having a polypeptide variant of this invention. An antibody or derivative of this invention can be made by co-expressing corresponding light and heavy chain CDRs-containing polypeptides in a suitable host cell by methods known in the art. See, e.g., Harlow and Lane, (1988) Antibodies; A Laboratory Manual, Cold Spring Harbor Laboratory, New York.

To make an antibody described herein, ASIC3 polypeptide or its antigenic fragment can be coupled to a carrier protein, such as KLH, mixed with an adjuvant, and injected into a host animal. Antibodies produced in that animal can then be purified by peptide affinity chromatography. Commonly employed host animals include rabbits, mice, guinea pigs, and rats. Various adjuvants that can be used to increase the immunological response depend on the host species and include Freund's adjuvant (complete and incomplete), mineral gels such as aluminum hydroxide, surface active substances such as lysolecithin, pluronic polyols, polyanions, peptides, oil emulsions, keyhole limpet hemocyanin, and dinitrophenol. Useful human adjuvants include BCG (bacille Calmette-Guerin) and Corynebacterium parvum.

Polyclonal antibodies, heterogeneous populations of antibody molecules, are present in the sera of the immunized subjects. Monoclonal antibodies, homogeneous populations of antibodies to a particular antigen, can be prepared using standard hybridoma technology. See, e.g., Kohler et al. (1975) Nature 256, 495; Kohler et al. (1976) Eur. J. Immunol.6, 511; Kohler et al., (1976) Eur. J. Immunol. 6, 292; and Hammerling et al. (1981) Monoclonal Antibodies and T Cell Hybridomas, Elsevier, N.Y. In particular, monoclonal antibodies can be obtained by any technique that provides for the production of antibody molecules by continuous cell lines in culture such as described in U.S. Pat. No. 4,376,110; the human B-cell hybridoma technique (Kosbor et al. (1983) Immunol Today 4, 72; Cole et al. (1983) Proc. Natl. Acad. Sci. USA 80, 2026) and the EBV hybridoma technique (Cole et al. (1983) Monoclonal Antibodies and Cancer Therapy, Alan R. Liss, Inc., pp. 77-96). Such antibodies can be of any immunoglobulin class including IgG, IgM, IgE, IgA, IgD, and any subclass thereof. The hybridoma producing the monoclonal antibodies of the invention may be cultivated in vitro or in vivo. The ability to produce high titers of monoclonal antibodies in vivo makes it a particularly useful method of production.

A polynucleotide containing a nucleic acid sequence encoding an inhibitor of ASIC3 can be used to treat insulin resistance. The nucleic acid sequence can encode the above-described polypeptide, an anti-ASIC3 antibody, an anti-sense RNA, or a small interference RNA (e.g., an RNAi agent) that targets the ASIC3 and inhibits its expression or channel activity.

The term “RNAi” or “RNA interference” refers to a sequence-specific or selective process by which a target molecule (e.g., a target gene, protein or RNA) is down-regulated. Within the scope of this invention is utilization of RNAi featuring degradation, of RNA molecules (e.g., within a cell). Degradation is catalyzed by an enzymatic, RNA-induced silencing complex (RISC). RNAi occurs in cells naturally to remove foreign RNAs (e.g., viral RNAs). Natural RNAi proceeds via fragments cleaved from free double-stranded RNA, which directs the degradative mechanism. Alternatively, RNAi can be initiated by the hand of man, for example, to silence the expression of target genes.

The term “RNAi agent” refers to an RNA (or analog thereof), having sufficient sequence complementarity to a target RNA (i.e., the RNA being degraded) to direct RNAi. A RNA agent having a “sequence sufficiently complementary to a target RNA sequence to direct RNAi” means that the RNA agent has a sequence sufficient to trigger the destruction of the target RNA by the RNAi machinery (e.g., the RISC complex) or process. A RNA agent having a “sequence sufficiently complementary to a target RNA sequence to direct RNAi” also means that the RNA agent has a sequence sufficient to trigger the translational inhibition of the target RNA by the RNAi machinery or process. A RNA agent can also have a sequence sufficiently complementary to a target RNA encoded by the target DNA sequence such that the target DNA sequence is chromatically silenced. In other words, the RNA agent has a sequence sufficient to induce transcriptional gene silencing, e.g., to down-modulate gene expression at or near the target DNA sequence, e.g., by inducing chromatin structural changes at or near the target DNA sequence. The term “RNA” or “RNA molecule” or “ribonucleic acid molecule” refers to a polymer of ribonucleotides. The term “DNA” or “DNA molecule” or deoxyribonucleic acid molecule” refers to a polymer of deoxyribonucleotides. DNA and RNA can be synthesized naturally (e.g., by DNA replication or transcription of DNA, respectively). RNA can be post-transcriptionally modified. DNA and RNA can also be chemically synthesized. DNA and RNA can be single-stranded (i.e., ssRNA and ssDNA, respectively) or multi-stranded (e.g., double-stranded, i.e., dsRNA and dsDNA, respectively).

The polynucleotide can be delivered by the use of polymeric, biodegradable microparticle or microcapsule delivery devices known in the art. Another way to achieve uptake of the nucleic acid is using liposomes, prepared by standard methods. The polynucleotide can he incorporated alone into these delivery vehicles or co-incorporated with tissue-specific antibodies. Alternatively, one can prepare a molecular conjugate composed of a plasmid or other vector attached to poly-L-lysine by electrostatic or covalent forces. Poly-L-lysine binds to a ligand that can bind to a receptor on target cells (Cristiano, et al., 1995, J. Mol. Med. 73:479). Alternatively, tissue specific targeting can be achieved by the use of tissue-specific transcriptional regulatory elements that are known in the art. Delivery of “naked DNA” (i.e., without a delivery vehicle) to an intramuscular, intradermal, or subcutaneous site is another means to achieve in vivo expression.

In the above-mentioned polynucleotides, e.g., expression vectors, the nucleic acid sequence encoding an inhibitor of ASIC3 is operatively linked to a promoter or enhancer-promoter combination. Suitable expression vectors include plasmids and viral vectors such as herpes viruses, retroviruses, vaccinia viruses, attenuated vaccinia viruses, canary pox viruses, adenoviruses and adeno-associated viruses.

As is well known in the art, the dosage for a patient depends upon various factors as described above. Dosages will vary, but a preferred dosage for administration of polynucleotide is about 10⁶ to 10¹² copies of the polynucleotide molecule. This dose can be repeatedly administered as needed. Routes of administration can be any of those listed above.

Also within the scope of the invention is a packaged product including a container, an effective amount of an ASIC3 inhibitor and a legend associated with the container and indicating administration of the inhibitor for treating a subject suffering from or being at risk for developing insulin resistance. The inhibitor can be admixed with a pharmaceutically acceptable carrier, including a solvent, a dispersion medium, a coating, an antibacterial and antifungal agent, and an isotonic and absorption-delaying agent.

The inhibitor can be formulated into dosage forms for different administration routes utilizing conventional methods. For example, it can be formulated, in a capsule, a gel seal, or a tablet for oral administration. Capsules can contain any standard pharmaceutically acceptable materials such as gelatin or cellulose. Tablets can be formulated in accordance with conventional procedures by compressing mixtures of the inhibitor with a solid carrier and a lubricant. Examples of solid carriers include starch and sugar bentonite. The inhibitor can also be administered in a form of a hard shell tablet or a capsule containing a binder, e.g., lactose or mannitol, a conventional filler, and a tableting agent. The inhibitor can be administered via the parenteral route. Examples of parenteral dosage forms include aqueous solutions, isotonic saline or 5% glucose of the active agent, or other well-known pharmaceutically acceptable excipient. Cyclodextrins, or other solubilizing agents well known to those familiar with the art, can be utilized as pharmaceutical excipients for delivery of the therapeutic agent. Further, the inhibitor can be injected directly to the striatum via brain operation.

The efficacy of the inhibitor can be evaluated both in vitro and in vivo. For example, the inhibitor can be tested for its ability to repress gene expression or channel activity of ASIC3 in vitro. For in vivo studies, the inhibitor can be injected into an animal (e.g., an animal model) and its effects on insulin resistance are then accessed. Based on the results, an appropriate dosage range and administration route can be determined.

The above-described ASIC3 inhibitors, e.g., APETx2, are particular useful for acute treatment of insulin resistance, i.e., within one hour of food indigestion. For examples, a subject can receive 14 mg/kg of salicylic acid or 0.23 mg/kg of APETx2.

The above-described ASIC3 inhibitor, e.g., APETx2, can also be used in treating other diseases (e.g. obesity) that are associated with abnormally high level of ASIC3 gene expression or channel activity. A subject to be treated can be identified by methods known in the art or by determining the gene expression or channel activity level of the ASIC3 polypeptide in a sample prepared from a subject as described above. If the gene expression or channel activity level of the ASIC3 polypeptide is higher in the sample from the subject than that in a sample from a normal person, the subject is a candidate for treatment with an effective amount of an ASIC3 inhibitor.

The specific example below is to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. Without further elaboration, it is believed that one skilled in the art can, based on the description herein, utilize the present invention to its fullest extent. All publications recited herein are hereby incorporated by reference in their entirety.

EXAMPLE

Experimental Procedures

Mice

ASIC3−/− mice were generated as described in Chen et al., Proc. Natl. Acad. Sci. USA 99, 8992-8997, 2002. Congenic 129 mice were derived from breeding ASIC3 chimera with female 129S2/SvPasCrl wild type mice. All mice were raised in a 12:12-h light-dark cycle at 25° C. and 40% to 70% humidity. All experiments involved use of female mice in different age groups. The animals were bred in a specific pathogen-free facility and cared for in accordance with the Guide for the Use of Laboratory Animals (National Academy Press, Washington D.C.). The experimental protocol was approved by the animal use committee of Academia Sinica.

Primary Adipose Cell Culture and Immunostaining

Fat pads of periovarian white adipose tissue (WAT) were isolated from female mice. One gram of the isolated fat pad was digested with 2 mg/ml of collagenase type VIII (Sigma) in 4 ml Dulbeceo's modified Eagle's medium (DMEM) at 37° C. for 1 hour. Then, the digested tissues were centrifuged at 200xg for 5 minutes at 4° C. The supernatant, which contains mature adipocytes, was collected for RNA isolation. The cell pellet, the stromal-vascular fraction (SVF), consisted of mesenchynal cells (adipoblasts) and immature adipocytes (adipocyte precursors and postadipocytes) (Van, The adipocyte precusor cell. In: New perspectives in adipose tissue:structure, function, and development. Butterworths, Borough Green, England, chapt. 15, 353-382, 1985). The SVFs were collected for RNA isolation or re-suspended for cell culture in DMEM containing 10% fetal calf serum and antibiotics. SVF cells were plated at a concentration of 10⁵ cells per glass cover slide (12 mm in diameter) and maintained at 37° C. in 5% humidified carbon dioxide. The cultured cells then underwent whole-cell patch recording in 5 days.

For immunostaining, the cultured cells were fixed with icy methanol for 5 minutes, permeablized in 0.25% Triton X-100 for 5 minutes, and washed with PBS. The fixed cells were placed in a blocking solution containing 2% BSA, 0.2% Triton X-100 in PBS for 1 hour and incubated in a primary antibody solution (containing guinea pig anti-ASIC3 1:250 from Neuromics; rabbit anti-ACRP30 1:200 from Chemicon) overnight at 4° C. Then, the cells were washed 3 times for 3 minutes each in PBS and incubated with secondary antibodies diluted 1:200 in a blocking solution for 1 hour. Secondary antibodies were goat anti-guinea conjugated to Alexa Fluor 488 and anti-rabbit conjugated to Alexa Fluor 594 (Molecular Probes).

Real-Time Quantitative RT-PCR

Total RNA was isolated from frozen tissue, samples or isolated adipose cell fractions using the RNeasy kit (Qiagen). Total complementary DNA (cDNA) was synthesized from the total RNA using Superscript III RT (Invitrogen). Real-time polymerase chain reaction of individual cDNAs involved SYBR green dye to measure duplex DNA formation with the ABI Prism 7700 Sequence Detection System (PE Applied Biosystems) and normalized to the expression of GAPDH RNA. All the SYBR green core reagents and AmpliTaq Gold polymerase were purchased from PE Applied Biosystems (Foster City, Calif.). The thermal cycling conditions were 95° C. for 10 minutes, followed by 40 cycles of 95° C. for 15 seconds and 60° C. for 1 minute. The primers used in the real-time RT-PCR were

ASIC3 sense, 5′-TTTCACCTGTCTTGGCTCCT-3′ (SEQ ID NO:10) ASIC3 anti-sense, 5′-CAGGATAGTGGTGGGGATTG-3′ (SEQ ID NO:11) GAPDH sense, 5′-GGAGCCAAACGGGTCATCATCTC-3′ (SEQ ID No:12) GAPDH anti-sense, 5′-GAGGGGCCATCCACAGTCTTCT-3′. (SEQ ID NO:13) Electrophysiology of Adipose Cells

Whole cell recordings were made of primary culture adipose cells using a Multiclamp 700B amplifier (Axon instruments). Patch electrodes were pulled from thin-filament glass of 1.5 mm o.d. (Warner Instrument, Hamden, Conn., USA), and filled with a solution containing (in mM) KCl 100, EGTA 10, MgCl₂5, HEPES 40, Na₂-ATP 2, and Na₃-GTP 0.3 (pH adjusted to 7.4 with KOH and osmolarity to 300-310 mOsm). The electrodes had resistances of 2-5 MΩ when filled with this solution. Standard external solutions contained (in mM) NaCl 130, KCl 5, MgCl₂1, CaCl₂2, glucose 10, and Hepes 20 (pH 7.4 adjusted with NaOH). Once whole cell recordings were obtained, recordings were made in a Voltage-clamp mode, and ASIC3-mediated current was elicited by acid (pH 5.0 adjusted by MES) applied through a gravity-controlled perfusion system. Signals were low-pass filtered at a corner frequency of 3 kHz and then digitized online at a frequency of 10 kHz with use of a CED 1401 interface (Cambridge Electronic Design, The Science Park, Cambridge, UK) running Signal software provided by CED.

Single-Cell RT-PCR

After patch recordings, the adipose cells were picked up from the recording pipette and underwent single-cell RNA isolation. Total RNA of a single adipose cell was isolated by use of the Absolutely Nanoprep Kit (Strategene) according to the manufacture's guide.

The resulting total RNA was reverse-transcribed using Superscript III RT (Invitrogen). Then the quality of a small portion (1/60) of the resulting RT product underwent PCR to detect the expression of GAPDH RNA. One-sixth of the total RT solution was used as a template for multiplex PCR, which included the primer pair sets either for the ASIC family (including 7 ASIC subtypes, TRPV1, substance P, and CGRP), or adipose markers (including FAS, FGF10, aP2, DLK-1, SMAF1, ETO, UCP1, UCP3, ACRP30, adponectin receptor 1 & 2 (AdipoR1 and R2), PPARγ, and PPARγ2. The thermal cycling condition of multiplex PCR was 95° C. for 10 minutes, followed by 35 cycles of 95° C. for 15 seconds and 60° C. for 1 minute. The PCR products were the template (1/50volume) for nested PCR amplification to examine whether a specific gene was expressed in a single cell. Another set of primers, nested primers, were designed for the above-mentioned genes. The thermal cycling condition of the nested PCR was the same as for the multiplex PCR. After nested PCR amplification, the PCR products were analyzed in 4% agarose gel. The nested primers were

ASIC1a sense 5′-GCCAACTTCCGTAGCTTCAAG-3′ (SEQ ID NO:14) ASIC1a antisense 5′- TACCGCGTGAAGACCACTTTG-3′ (SEQ ID NO:15) ASIC1b sense 5′-TGCTCGGGTTGGATGAGAGTG-3′ (SEQ ID NO:16) ASIC1b antisense 5′-GGAGCAATAGAGCAGCATGTC-3′ (SEQ ID NO:17) ASIC2a sense 5′-CCGAAGCAGTTCAGCATGCTGGAG-3′ (SEQ ID NO:18) ASIC2a antisense 5′-ATCCTCGCCTGAGTTAAACATG-3′ (SEQ ID NO:19) ASIC2b sense 5′-TGCCGCCGCGCCACTTCGAGG-3′ (SEQ ID NO:20) ASIC2b antisense 5′-ATCCTCGCCTGAGTTAAACATG-3′ (SEQ ID NO:21) ASIC3 sense 5′-TCTGGCAACACTCTGCTCCAGGAAG-3′ (SEQ ID NO:22) ASIC3 antisense 5′-ACTGGGAGCGGTAGGAGGCCTG-3′ (SEQ ID NO:23) ASIC4 sense 5′-GGGTGACACAGGACAGTCAG-3′ (SEQ ID NO:24) ASIC4 antisense 5′-CAGCCAAGGTCTGAAAGGTC-3′ (SEQ ID NO:25) ASIC5 sense 5′-CCCTGGTTTTGCTGATGCTG-3′ (SEQ ID NO:26) ASIC5 antisense 5′-TTCCCACAGGAGAAGACAAAC-3′ (SEQ ID NO:27) TRPV1 sense 5′-TCCACTGGTGTTGAGACGCC-3′ (SEQ ID NO:28) TRPV1 antisense 5′-CTTTGAACTCGCTGTCAGTC-3′ (SEQ ID NO:29) Substance P sense 5′-GTGACCTCCCCAAAAGTAGA-3′ (SEQ ID NO:30) Substance P sense antisense 5′-ACAGGAGTCTCTGCTTCCAG-3′ (SEQ ID NO:31) CGRP sense 5′-GCCACCTGTGTGACTCATCG-3′ (SEQ ID NO:32) CGRP antisense 5′-GGCTTCAGAGCCCACATTGG-3′ (SEQ ID NO:33) FAS sense 5′-ATTGCATCAAGCAAGTGCAG-3′ (SEQ ID NO:34) FAS anntisense 5′-GAGCCGTCAAACAGGAAGAG-3′ (SEQ ID NO:35) FGF10 sense 5′-CTGGAAAGCACTTGGGTCAT-3′ (SEQ ID NO:36) FGF10 antisense 5′-GGAGACAGAATGCACAAGCA-3′ (SEQ ID NO:37) aP2 sense 5′-ACGACAGGAAGGTGAAGAGC-3′ (SEQ ID NO:38) aP2 antisense 5′-AAATTTCCATCCAGGCCTCT-3′ (SEQ ID NO:39) DLK-1 sense 5′-CTAACCCATGCGAGAACGAT-3′ (SEQ ID NO:40) DLK-1 antisense 5′-CTTGCACAGACACTCGAAGC-3′ (SEQ ID NO:41) SMAF1 sense 5′-CTGAGTGTGGCTGTGAGGAG-3′ (SEQ ID NO:42) SMAF1 antisense 5′-CAGGTCTGACAACGGGAGAT-3′ (SEQ ID NO:43) ETO sense 5′-CCTGTCAATCCAGACCCAGT-3′ (SEQ ID NO:44) ETO antisense 5′-TGTCATGGGCTTTCCTCTCTG-3′ (SEQ ID NO:45) UCP-1 sense 5′-GGCCCTTGTAAACAACAAAATAC-3′ (SEQ ID NO:46) UCP-1 antisense 5′-GGCAACAAGAGCTGACAGTAAAT-3′ (SEQ ID NO:47) UCP-3 sense 5′-ACTCCAGCGTCGCCATCAGGATTCT-3′ (SEQ ID NO:48) UCP-3 antisense 5′-TAAACAGGTGAGACTCCAGCAACTT-3′ (SEQ ID NO:49) ACRP30 sense 5′-GTTGCAAGCTCTCCTGTTCC-3′ (SEQ ID NO:50) ACRP30 antisense 5′-TCTCTCCAGGAGTGCCATCT-3′ (SEQ ID NO:51) AdipoR1 sense 5′-TCTCCTGGCTCTTCCACACT-3′ (SEQ ID NO:52) AdipoR1 antisense 5′-CCACAATGATGGCAGAGATG-3′ (SEQ ID NO:53) AdipoR2 sense 5′-TGGAGGCTGTTGGTAGTGAG-3′ (SEQ ID NO:54) AdipoR2 antisense 5′-TCTTAGGGAACCGAATCACC-3′ (SEQ ID NO:55) PPARγ sense 5′-GGAATCAGCTCTGTGGACCT-3′ (SEQ ID NO:56) PPARγ antisense 5′-TGGGTCAGCTCTTGTGAATG-3′ (SEQ ID NO:57) PPARγ2 sense 5′-CTCCTGTTGACCCACAGCATG-3′ (SEQ ID NO:58) PPARγ2 antisense 5′-GTGGAGCAGAAATGCTGGAG-3′. (SEQ ID NO:59) Metabolic Studies

Mice of 9, 16, or 25 weeks old were tested for glucose tolerance (GTT) and those of 18 or 27 weeks old for insulin tolerance (ITT). For GTT, the mice fasted overnight then were injected with glucose intraperitoneally at 1.5 mg/g body weight. For ITT, the mice were given free access to food and injected with insulin intraperitoneally at 0.5 mU/g body weight. The glucose levels were measured in blood withdrawn from the tail vein. To study effects of ASIC3 blockers, salicylic acid (14 mg/kg) or APETx2 (0.23 mg/kg) were injected with glucose or insulin intraperitoneally.

Histology and Morphometric Analysis of Tissues

Periovarian WAT was isolated from each mouse and fixed in the Bouin's solution (Sigma) at room temperature for 24 hours. Then, the fixed tissues were embedded in paraffin, sectioned in 5-μm sections, and stained with haematoxylin and eosin. Morphometric analysis of WAT from 1000 or more cells from 3 to 4 different animals per age group and per genotype involved use of MetaMorph Offline software (Universal Imaging Corporation).

Results

Inhibition of ASIC3 Enhances Insulin Sensitivity that is Age-Dependent

In the present study, it was found that ASIC3−/− mice had a significantly smaller body size than ASIC3+/+ mice and that the ASIC3−/− mice, as they aged, did not build up body fat mass as did ASIC3+/+ mice (n=10-22, p<0.05). The cells of both white and brown adipose tissues in 8-week-old ASIC3−/− mice did not differ in appearance from those of ASIC3+/+ mice. But at 25 weeks, the white adipose tissues of ASIC3−/− mice were full of small adipocytes and some contained multilocular lipid accumulation. In contrast, the brown adipose tissues appeared the same as those of the ASIC3+/+ mice. The mean cell size of adipocytes of 25-week-old ASIC3−/− mice (1018±20 μm²) was much smaller than that of the same-aged ASIC3+/+ mice, as well as 8-week-old mice (1877±39 μm² in ASIC3+/+ and 1897±41 μm² in ASIC3−/−).

Because the small adipocytes usually reflect increased insulin sensitivity (Um et al., Nature 431, 200-205, 2004 and Komazawa et al., Nat. Medicine 10, 1208-1215, 2004), experiments were conducted to compare the responses to glucose tolerance test (GTT) and insulin tolerance test (ITT) between the ASIC3−/− and ASIC3+/+ mice (n=6-9 in each group). As the animals aged, the ASIC3−/− mice showed better glucose tolerance and insulin sensitivity than the ASIC3+/+ mice (P<0.05, Mann-Whitney test). In contrast, glucose tolerance and insulin sensitivity in younger mice (from 9 to 18 weeks) did not differ between the 2 genotypes. However, older ASIC3+/+ mice (25 weeks old; n=6) showed significantly poorer results on glucose tolerance test and were more glucose intolerant than younger mice (9 or 16 weeks old; n=6 or 7) (P<0.05). ASIC3+/+ mice of 27 weeks old only had subtle changes in ITT. Such age of ASIC3−/− mice showed no impaired glucose tolerance but, rather improved insulin sensitivity. Put differently, ASIC3−/− mice did not develop age-dependent impaired glucose tolerance (i.e., glucose intolerance). Surprisingly, older ASIC3−/− mice showed better results on the ITT than young mice (n=6, *P<0.05). Compared with ASIC3+/+ mice, ASIC3−/− mice seemed to maintain normal glucose metabolism by improving insulin action with age.

To test whether ASIC3 selective blockers could reverse age-dependent glucose intolerance and insulin resistance, we tested the effects of salicylic acid and APETx2 in the aging mouse groups. Salicylic acid and APETx2 are selective antagonists for ASIC3 in different kinetics (Diochot et al., EMBO J. 23, 1516-1525, 2004 and Voille et al., J. Neurosci. 21, 8026-8033, 2001). It was found that treatment with high dosage salicylic acid could reverse in part the age-dependent effect of glucose tolerance but not affect insulin tolerance test of ASIC3+/+ mice. In contrast, the specific peptide inhibitor of ASIC3, APETx2, could effectively reverse the age-dependent effect of glucose tolerance and significantly enhance the insulin sensitivity in aging ASIC3+/+ mice, as seen on ITT. The differential effects of salicylic acid may be due to the different kinetics in blocking ASIC3 from APETx2 or could be due to an inhibitory effect on IKKβ. Continual treatment with salicylate (120 mg/kg/day) in rodents disrupted IKKβ activity and reversed obesity- and diet-induced insulin resistance by sensitizing insulin signaling.

Age-Dependent Glucose Tolerance is Associated with ASIC3 Expressed in WAT

To examine why the deletion of ASIC3 showed a profound effect in adipocytes, we tested whether ASIC3 is expressed in white adipose tissue (WAT). PCR results revealed the expression of ASIC1b, ASIC3, ASIC4 and ASIC5. In primary WAT culture, we detected the immunoreactivity of ASIC3 in some adipocytes. In young ASIC3+/+ mice (9 weeks old), ASIC3 was evenly expressed in the mature adipocyte fraction and cells of stromal-vascular fraction (SVF) isolated from WAT. Interestingly, WAT-expressed ASIC3 mRNA of the SVF but not mature adipocytes was significantly upregulated (by ˜11-fold) in aged mice (30 weeks). In contrast, the expression of ASIC3 in aged dorsal root ganglia (DRG) was only one third of its expression in young DRG.

WAT-Expressed ASIC3 is a Functional Ion Channel

To determine whether WAT-expressed ASIC3 is functional, we applied whole-cell patch recording to examine the acid-evoked currents in cultured SVF cells isolated from mice 8 to 10 weeks old. It was found that acid (pH 5.0) indeed evoked inward currents (sustained or biphasic) in SVF adipose cells. The acid-induced currents were sensitive to selective ASIC3 blockers, salicylic acid and APETx2. 8 adipocytes and 71 stroma-like cells were tested. It was found that acid stimulation elicited an inward current in 5 adipocytes and 40 stroma-like cells. The acid-sensitive adipose cells varied in morphology, cell size (ranging from 15.6 to 64.3 μm in diameter), and kinetics of acid-induced currents. Some of the cells exhibited biphasic currents while others had sustained currents. The electrophysiological properties of the cells with biphasic current differed from those with sustained current in many aspects, including capacitance (35.8 vs 83.7 pF), peak amplitude (314.1 vs 57.6 pA), current density (8.1 vs 2.5 pQ/pF), and rising time (33.6 vs 885.8 ms). The acid evoked biphasic current was found only in stroma-like cells.

No action potential could be induced among these adipose cells, which indicates that they were not contaminated from enteric ganglia or smooth muscle cells. Table 1 below summarizes the cell sizes and electrophysiological properties of adipose cells, comparing cells with sustained currents and cells with biphasic currents. ** P <0.01, t test.

TABLE 1 Peak Current Diameter Vm Capacitance amplitude density Rising time (μm) (mV) (pF) (pA) (pQ/pF) (ms) Sustained 28.2 ± 2.3 −18.6 ± 2.3 83.7 ± 10.6  57.6 ± 10.6 2.5 ± 0.4 885.8 ± 104.7 (n = 31) Biphasic 23.8 ± 2.2 −14.6 ± 2.9 35.8 ± 4.4** 314.1 ± 63.2** 8.1 ± 2.3**  33.6 ± 10.1** (n = 14) No current 24.1 ± 1.6 −13.6 ± 3.4 37.6 ± 3.7 (n = 34)

The WAT-expressed ASIC3 was mainly composed of ASIC3 homomeric channels, as indicated by single-cell RT-PCR after whole-cell patch recording. ASIC3-ASIC1b and ASIC3-ASIC2b heteromeric channels were found in a small portion (3/14 ) of the adipose cells. The acid-sensitive adipose cells also expressed many adipose cell markers, including FAS, FGF10, aP2, ETO, PPARγ, adipoR1, and adipoR2. AdipoR2 was the only marker expressed in all types of cells with acid-induced currents. Surprisingly, 3 cells without acid-evoked inward currents also expressed ASIC3, indicating that the cells may need additional factors to support ASIC3 to form a functional channel. Table 2 below summarizes molecular identity of single-cell RT-PCR in cells with or without acid-evoked inward currents. The adipose cells were sub-grouped according to the expression of ASIC3 homomeric or heteromeric channel. The bracketed numbers indicate cell number that expresses each adipocyte gene marker.

TABLE 2 Acid-evoked ASIC Cell inward current gene Adipocyte markers and other genes number Sustained ASIC3 CGRP (1), FAS (2), FGF10 (1), 7 aP2 (1), ETO (1), ACRP30 (1), PPARγ (1), AdipoR1 (2), AdipoR2 (3) ASICIb, aP2 (1), AdipoR2 (1) 2 2 ASIC3 Biphasic ASIC3 CGRP (1), FAS (1), FGF10 (2), 4 AdipoR1 (1), AdipoR2 (1), PPARγ (1) ASIC2b, AdipoR2 (1) ASIC3 No current ASIC3 FAS (1) 1 ASICIb, FAS (1), FGF10 (1), PPARγ (1), 2 ASIC3 UCP3 (1)

The above results indicate that cells in adipose tissue expressed functional ASIC3 channels that may contribute to age-dependent glucose intolerance and insulin resistance. The deletion or inhibition of the channel increased insulin sensitivity in ageing mice. ASIC3−/− mice were smaller in size than ASIC3+/+ mice and accompanied by age related beneficial phenotypes: smaller visceral fat mass, smaller adipocyte size, and better profiles in GTT and ITT. These beneficial phenotypes of ASIC3−/− mice were not due to the diet, as food intake was normal in these mice. In contrast, ageing led to glucose intolerance in ASIC3+/+ mice, which was associated with the upregulation of WAT-expressed ASIC3. ASIC3 signaling in adipose cells but not primary sensory afferents is probably responsible for the age-dependent glucose intolerance because the expression of ASIC3 in dorsal root ganglia declined with age.

ASIC3 is known as the most sensitive acid-sensing ion channel (pH_(0.5) activation ˜6.7) and dominantly expressed in primary sensory neurons, especially in metaboreceptive sensory neurons or ischemia-sensing neurons. The reasons why the proton sensor ASIC3 mediates signals in adipose tissue may lie in lactate.

Lactic acid produced by anaerobic metabolism is a potent enhancer for acid-sensing ion channels (Immke and MacCleskey, Nat. Neurosci. 4, 869-870, 2001).

Apart from exhausted muscle or ischemic tissues, adipose tissue, especially in old and fat individuals, actively produces lactate as a metabolic product of glucose. Large fat cells from old, fat rodents convert 40% to 50% of the total glucose taken up in lactate as compared with only 5% to 15% in young ones. However, the role of adipose lactate is still ambiguous, because the traditional concept of gluconeogenesis cannot explain how it is actively produced in old, obese subjects. Therefore, lactate may be a signaling compound that coordinates cell and systemic function. Because elevated basal level of lactate is consistently found in diabetic subjects who also display marked insulin resistance, the adipose ASIC3 activity maybe the missing link in the mediation of lactate signaling. ASIC3 responds better to lactic acid than to other acids to depolarize ischemia-sensing neurons. Therefore, these neurons might function as metaboreceptors to sense the anaerobic metabolism of tissues and trigger acid-linked pain sensation in muscle and

Heart (Chen et al., 2002). As lactate does not cause pain in many other tissues (e.g., adipose tissues), ASIC3 may function not only as a pain sensor but also in monitoring the metabolic state of tissues. ASIC3 channels open when pH drops from 7.4 to 7.0 and lactate can enhance the opening of ASIC3 at near-physiological pH 7.4 (Immke and McCleskey, Neuron 37, 75-84, 2003). With the ability to detect lactic acidosis, WAT-expressed ASIC3 might be constantly active in adipose tissue of elderly and obese subjects, in whom approximately 50% of total glucose is metabolized to lactate. The tonic signal mediated by WAT-expressed ASIC3 may hamper the glucose uptake by cells and result in the development of insulin resistance with age, when the WAT expression of ASIC3 is also increased. With ASIC3 deficiency, the adipose tissues cannot properly detect lactate and switch the cell fate to enhance insulin sensitivity.

Ageing can result in high levels of circulating lactate and reduce insulin sensitivity. With physiological ageing, resistance to the action of insulin and decline in glucose tolerance develop, resulting in a high prevalence of type 2 diabetes in older people. Type 2 diabetes is the most common metabolic disease in ageing population worldwide and closely associated with mitochondria dysfunction. Because mitochondria dysfunction switches cells to anaerobic glucose metabolism and increases lactate production, it is likely that the lactate sensor, ASIC3, plays a role in control of the age-dependent glucose metabolism.

Human ASIC3 resides within the chromosomal region 7q36 (de Weille et al., FEBS Lett. 433, 257-260, 1998), which was one of the identified quantitative trait loci for fasting glucose, insulin, and insulin resistance (An et al., Diabetes 54, 909-914, 2005). This fact also supports the role of ASIC3.

In sum, the above results suggest that blocking the WAT-expressed ASIC3 signaling is useful in the control of age-dependent insulin resistance and type 2 diabetes.

OTHER EMBODIMENTS

All of the features disclosed in this specification may be combined in any combination. Each feature disclosed in this specification may be replaced by an alternative feature serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features.

From the above description, one skilled in the art can easily ascertain the essential characteristics of the present invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, other embodiments are also within the scope of the following claims. 

1. A method of identifying a compound for treating insulin resistance, the method comprising: contacting a first in vitro cell expressing an acid-sensing ion channel 3 with a test compound, and determining an expression or activity level of the acid-sensing ion channel 3 in the cell, and identifying the test compound as a candidate compound for treating insulin resistance if the expression or activity level is lower than that determined in the same manner from a second in vitro cell except that the second cell is not contacted with the compound.
 2. The method of claim 1, wherein the cell is an adipose tissue cell.
 3. The method of claim 2, wherein the adipose cell is a white adipose tissue cell.
 4. The method of claim 1, wherein the activity of the acid-sensing ion channel 3 is determined by electrophysical analysis.
 5. The method of claim 1, wherein the ion channel activity of the acid-sensing ion channel 3 is determined by whole-cell patch recording, a voltage-sensitive dye, an ion-sensitive dye, or a cytotoxicity assay.
 6. The method of claim 1, wherein the method further includes administering the candidate compound to a non-human animal to confirm an efficacy thereof to treat insulin resistance.
 7. The method of claim 1, wherein the first cell or second cell is non-neuronal and AdiproR2+.
 8. The method of claim 7, wherein the first cell or second cell is selected from the group consisting of NIH3T3 cells, NIH3T3 L1 cells, differentiated NIH3T3 L1 cells, adipose cells, and fibroblast cells.
 9. The method of claim 8, wherein the first cell or second cell is positive for one or more of FAS, FGF10, aP2, ETO, PPARγ, and AdipoR1.
 10. A method of identifying a compound for treating insulin resistance, the method comprising: contacting a first cell expressing an acid-sensing ion channel 3 with a test compound, and determining an expression or activity level of the acid-sensing ion channel 3 in the cell, and identifying the test compound as a candidate compound for treating insulin resistance if the expression or activity level is lower than that determined in the same manner from a second cell except that the second cell is not contacted with the compound, wherein the compound is selected from the group consisting of proteins, peptides, peptidomimetics, peptoids, antibodies, small molecule compounds, and drugs. 